Transport and pathways of leucine and glutamine degradation were evaluated in resting human peripheral lymphocytes and compared with the changes induced by concanavalin A (ConA). Cells were incubated with [1-14C]leucine (0.15 mM), [U-14C]leucine (0.15 mM), or [U-14C]glutamine (0.4 mM) after culture with or without 2, 5, 7, or 10 micrograms/ml ConA for 2, 18, or 24 hours, respectively. Initial rates of transport of leucine and glutamine were augmented 2.7-fold and threefold by the mitogen. Leucine transamination, irreversible oxidation, and catabolism beyond isovaleryl-CoA were increased by 90%, 20%, and 60%, respectively. Glutamine utilization increased threefold; accumulation of glutamate, aspartate, and ammonia increased by 700%, 50%, and 100%, respectively, and 14CO2 production by about 400% in response to ConA. The results indicate that ConA stimulates to about the same extent transport of leucine and glutamine into lymphocytes. Glutamine is mainly channeled into catabolic pathways, while leucine remains largely preserved. It is suggested that these metabolic changes provide more leucine for incorporation into protein and more N- and C-atoms required for the synthesis of macromolecules and energy from glutamine.